Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!kwe
From: kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England))
Newsgroups: comp.dcom.lans
Subject: Re: Request For Opinions:  Optical Fiber Physical Topologies
Summary: consider a multi-hub star topology
Message-ID: <29505@bu-cs.BU.EDU>
Date: 12 Apr 89 18:48:33 GMT
References: <4824@charon.unm.edu>
Organization: Boston U. Information Technology
Lines: 120

In article <4824@charon.unm.edu>, dd@ariel.unm.edu writes:
> 
> My question, addressed to anyone who has a solid argument, but
> particularily to folks who might have installed an FDDI compatible
> wiring plant is:  What is the best way to go?  Should I build a ring,
> or a star-shaped ring, or a ring of rings, or a ring of stars, or
> what?  I realize that the ultimate decision will depend on the
> particulars of our campus, but if anyone can think of factors that I
> ought to consider, please let me know.
> 
> Thank you all for your help!
> 
> Don Doerner				dd@ariel.unm.edu

	[This turned out rather long.]

	We at Boston University installed Pronet-80 in '87 on a fiber
optic cable plant and we are writing the contract for the contractor
to install a new fiber plant in new conduit to extend our fiber
network for Pronet-80 and other uses.

	Perhaps what we did in our pilot installation and what we
designed for the extension would be instructive.

	As you know, Pronet-80 is essentially the same as FDDI with
the important exception that FDDI has Station Management.  Pronet-80
has no management of any kind.  The cable plants are the same.  Other
uses for fiber include f/o Ethernet and sometime in the future,
telephone services might be useful on campus over fiber.  I am
including single mode fiber in the entire cable plant so I can
distribute external services anywhere on campus as needed.

	When we designed our pilot cable plant, we installed the fiber
in a big ring around a street in our campus.  We installed patch
panels in the basement of each building and we ran 24 fibers between
each patch panel.  We could install a f/o interface in each building
and the distance between f/o interfaces would be a short hop on the
trunk between panels.  Of course, for those buildings with no f/o
service, we patched through the panel to get to the next panel.  I
think we have about a dozen patch panels, but only seven routers.
That means that there are a lot of patch connections in some of our
links and most of our loss budget is in the connectors.  This is no
problem over a short distance.

	But then in the process of debugging a problem with the
receivers in the f/o interfaces, we ran up against the problem of too
much loss in some links and we had to go back and take out some patch
cords to reduce the loss.  Still not really a problem.

	Then we had a need to do some f/o Ethernet extension to solve
a problem we had to grandfather, so we needed some fiber with a
different topology.  Still workable, but getting a little kludgy.

	Now, scale up the distance, number of buildings, etc and you
can see that this topology design begins to get unworkable very fast.

	Our extension project triples the geographic scope of our
fiber cable plant with almost no expansion of our router base.  That
part of campus is less compute-intensive today, so the fiber is really
more for the future than to solve today's problems.  However, our
admin computing people will be joining this extension, so we will have
our hands full.  Anyway we have a big sparse network and the need for
some low budget networking.  Not every Mac cluster can afford a
FastPath and a $25k p4200 with Pronet-80.   :-)

	After much looking around at other designs and much thought,
etc I came up with the multihub star topology as our preferred
solution.  A full blown star was impractical because we didn't have a
logical place to put the hub and a ring topology just wasn't practical
in terms of providing low loss paths where we wanted them in our
sparse matrix.

	We defined three hub sites in our extension: one on each end
for service coverage and further extension, and one roughly in the
middle.  We joined one end hub to our pilot plant thru our computer
center. 

	The hubs "tile the plane".  That is, they each have a service
area that covers the whole extension area without overlap.  Each hub
serves from three to four buildings.  The building service is star
configured from each hub to each building in its service area.  We are
running twelve multimode fibers to each building.

	The center hub is linked to each of the end hubs with a dual
trunk cable, each cable contains 24 multimode and 4 single mode
fibers.  Unfortunately, I do not have widely separated paths to route
each of my trunk links thru, but I recommend physically dispersing
your trunk runs as much as possible.  I am having them routed through
different ducts in the bank and down each side of the vaults, etc to
maximize physical redundancy and fault tolerance.  You could tie your
hubs together in a ring or a matrix, depending on geography and number
of hubs.

	I will end up placing a router in each hub and they will be
linked on the unspliced, continuous trunk cables.  Then I can install
other routers or f/o ethernet extensions from the hubs to the
buildings in the service area.  I can place most anything in the hub
and link it up with fiber to any or every building in the service
area.  Maybe even PBXs.

	We designed a broadband extension with exactly the same
topology and a trunk amp in each hub.  All trunk service is in one of
three hub closets.  No equipment in any manhole.

	Now when we plug our f/o interfaces together we just have to
be sure that there is enough loss in each path so we don't have to buy
attenuators.  Our longest trunk run is 2800 feet, I think, so, again,
loss is determined by connectors and not fiber loss.

	I am sure we will like our multihub topology much better.  I
hope the trunk fibers last a while before we have to pull more.

	BTW, the broadband trunk pulls are going to be very tough,
requiring winches and all kinds of special work.  The fiber following
the same path can be pulled in by one man by hand.  You could probably
do it yourself.  Our broadband trunk is one-inch cable.  My tech is
very demanding about signal quality.  :-)  The fiber is about half an
inch, feather light and easily flexed.

	--Kent England, Boston University